In the area of automobile parts, domestic electric appliances, office automation devices, audio-video equipment, a industrial trend is observed to replace metal parts, e.g., such parts as metal gears or the like, and especially sliding parts, with plastic ones toward weight reduction of aforementioned commodities or toward its cost reduction. Examples of such replacements are combinations of plastic gears and metal gears used in reduction gears associated with wiper motors or electric power assisted steering units contained in steering mechanisms of vehicles. On the other hand, loads and sliding speeds of such mechanisms are increasing with a trend of miniaturization, and sliding conditions are getting severe toward higher transmission efficiency. For such industrial trend, the required performance of the lubricating grease composition is becoming higher and it is getting more difficult to accomplish its low friction property and durability enough to satisfy such required performance. Thus, it was proposed to use grease compositions that contain specific finely powdered polytetrafluoroethylene. The proposed composition is formed into a lubricating film between the interacting parts, and will restrain the wear of parts and improve the endurance life of the sliding parts even under the aforementioned stringent condition (see Patent Reference 1).
It should be noted that glass-fiber-reinforced plastics, such as glass-fiber-reinforced polyamides or other organic resins, possess excellent tensile strength, flexural modulus of elasticity, and other mechanical properties, and satisfy the aforementioned required performance for increase in loads and sliding speeds of such sliding plastic parts. However, even though the reinforced plastic parts are superior in mechanical strength to plastic parts without reinforcement, they are insufficient in long-term performance, and even the use of conventional grease compositions cannot protect these materials from degradation of its mechanical strength properties along time.
Patent Reference 2 and Patent Reference 3 disclose lubricating grease compositions which contain polyurea and calcium soap as thickeners, as well as tricalcium phosphate and calcium carbonate as extreme-pressure wear-resistant additives. Patent Reference 4 discloses a lubricating grease composition which contains tricalcium phosphate and mineral oil compound, and Patent Reference 5 discloses a lubricating grease composition which contains tricalcium phosphate and urea compound as a thickeners. Although such lubricating grease compositions show excellent performance applied for metal-to-metal pairs, they are insufficient to improve the sliding performance of plastic parts. More specifically, they do not show sufficient performance to improve the sliding properties of parts made of glass-fiber-reinforced plastics. And when the sliding pair is consisting of metal parts and such plastic parts (especially, glass-fiber-reinforced ones) noticeably damages the sliding surfaces on the metal counterparts. Aforementioned lubricating grease compositions of Patent references are insufficient to solve the above-described problems.    [Patent Reference 1] Japanese Unexamined Patent Application Publication (Kokai) 2001-89778    [Patent Reference 2] Kokai S64-26698    [Patent Reference 3] Kokai H04-41714    [Patent Reference 4] Kokai H04-65119    [Patent Reference 5] Kokai H08-157859 (equivalent to U.S. Pat. No. 4,743,671)